The present invention relates to a sheet feeder for use in a sheet-fed printing press having a paper feed cylinder on the paper feed side of an impression cylinder, or a multi-chromatic sheet-fed printing press having a single transfer cylinder or a plurality of transfer cylinders between the impression cylinders of adjacent two printing units. More particularly, the invention relates to a device for detecting and discharging an improperly fed sheet or sheets of paper, such as two or more simultaneously fed sheets, a diagonally oriented irregular sheet, or a dirty sheet, supplied to the paper feed cylinder or the transfer cylinder or cylinders.
Sheet-fed printing presses are designed such that they are normally supplied with one sheet of paper at a time during their operation. When two or more sheets of paper are simultaneously supplied, however, one or more sheets of paper remained unprinted and are discharged. Furthermore, when such improperly fed sheet or sheets are transferred into the printing press, they tend to cause trouble, damaging the blankets of the cylinders or resulting in a paper jam. Repairing the damaged parts or removing the paper jam requires a large expenditure of time and labor.
There is known a sheet-fed printing press having a means for detecting two or more simultaneously fed sheets of paper and stopping the feed of such sheets of paper. After the feed of the sheets has been stopped, the improperly fed sheets have to be removed. In case an irregularly fed sheet of paper which may be oriented vertically or laterally, or a dirty sheet of paper, is supplied, an improperly printed sheet of paper comes out of the printing press, and is discharged into a paper discharge section where it is mixed with other properly printed sheets of paper. Therefore, the improperly printed sheet has to be subsequently removed from the paper discharge section for desired quality control. Since the removal of the improperly printed sheet is timeconsuming and laborious, the rate of printing sheets of paper on the printing press is low.